1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a ring oscillator for calibrating a phase error between in-phase and quadrature-phase signals occurring in wireless communication systems and a phase-error calibration method therefor.
2. Description of the Related Art
In general, communication systems may carry and send data on both an in-phase channel (I-channel) and a quadrature-phase channel (Q-channel). Thus, such communication systems require both in-phase and quadrature-phase oscillation signals having a 90° phase difference therebetween in order to completely restore a desired signal.
Such oscillation signals are used as input signals to a down-converter for converting a signal down to a lower frequency band at a signal-receiving stage, and used as input signals to an up-converter for converting a signal up to a higher frequency band at a signal-transmitting stage. If the in-phase oscillation signal and the quadrature-phase oscillation signal do not form an exact 90° phase difference therebetween, the Bit Error Rate (BER) may become high when a signal is completely restored, so it is critical to generate signals having an exact phase difference therebetween.
In particular, a direct conversion receiver (DCR) that separates signals into both channels at high frequencies more severely exhibits the effect of I/Q phase mismatch. Thus, a phase error occurring between oscillation signals on both channels should be eliminated for precise operation of signal transmitters and receivers.
The methods for generating oscillation signals having a 90° phase difference therebetween may include the method (in a structure developed by Athena Semiconductors, Inc.) that uses a butterfly structure for calibrating a phase error of I/Q signals in a digital signal region and the method that uses Poly Phase Filters (PPFs) for calibrating a phase error of I/Q signals in an analog signal region.
The former structure of Athena Semiconductors, Inc. is for a method that measures the power of in-phase and quadrature-phase signals in order to determine an Rx gain error and measuring correlation of the in-phase and quadrature-phase signals in order to determine the Rx phase error, so as to calibrate the phase error, which has an advantage in that the butterfly structure can be used to calibrate the phase error of in-phase and quadrature-phase signals as well as a gain error.
The latter method does not need any extra circuits for calibrating the error of the in-phase and quadrature-phase signals, and can tune a phase in a wide range.
However, despite such advantages, the former structure of Athena Semiconductors, Inc. has a disadvantage due to calibration difficulties, long processing time due to complicated digital processing if the phase error of the in-phase and quadrature-phase signals becomes large, and requires the assumption that Rx calibrations are accomplished when the Tx calibrations are supposed to be perfect. Meanwhile, the latter method also has a problem in that it cannot be applied to systems that do not use PPFs.